Reproduction requires interaction between multiple systems, including the adipocyte hormone leptin, hormones of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes, and also immune function and social processes. We propose to explore these systems in a socially complex reproductive environment, using 2 species of seasonally breeding bats as models. Due to their lengthy gestation and lactation periods, highly seasonal breeding, high levels of glucocorticoids, and the high energetic demands of flight, these bats are ideal for studying energy balance and reproduction as mediated by leptin, the HPA axis, and the immune system. They are also good models for exploring social influences on physiology, the role of leptin in puberty and pregnancy, and potentially for understanding glucocorticoid resistance syndromes. This study aims, 1) to characterize leptin levels and HPA and MPG activity across the breeding season in both sexes and to explore the interactions between these systems, and 2) to examine social influences on reproduction (one species is much more social than the other) and variation in immune function. These goals will be achieved by observing social behavior and obtaining monthly blood samples across the breeding season to assess leptin, adrenocorticotropic hormone, glucocorticoids, and gonadal steroids and to perform complete blood counts. We will also determine differences in stress reactivity before vs. after infant birth, alterations in HPA physiology attendant to group formation, and leptin mRNA levels from fat biopsies collected pre- and postpartum and from the placenta.